Brush and brush holder maintenance often causes substantial down time expense, especially in the case of large dynamoelectric machines, such as locomotive DC traction motors. The length of the brushes is often limited by the holder design, and the harsh environment often contributes to premature failure of exposed portions of the holder and spring elements.
Various arrangements have been proposed for simplifying brush replacement and maintenance, but engineering design tradeoffs have typically limited the brush length, compromised the protection of the brush holding apparatus, or unduly increased the complexity of the assembly.